1. Field of the Invention
The present invention relates to a multi-stage screw compressor unit suitable for applications in the case the suction pressure or discharge pressure fluctuates widely when used for compressing and supplying gas for a refrigerating machine, air conditioner, gas turbine booster, natural gas pipe line, chemical process, spherical holder, etc. and the method of operation thereof.
2. Description of the Related Art
Capacity controllable screw compressors have been used widely for refrigerating machines. A plurality of compressors have been connected to compress gas through a plurality of stage, for example, two stages or three stages to reduce the compression ratio per one stage for improving compression efficiency, for polytropic efficiency is low if it is intended to attain high compression ratio (ratio of discharge pressure to suction pressure) by a single-stage compressor. As initial cost increases when a plurality of compressors are simply connected, compound compressor units in which a lower and a higher pressure stages are provided in a compressor unit driven by a driving machine have been prevailing to reduce initial cost.
Generally, in a screw compressor, the internal volume ratio is determined in the design stage, and a compressor of proper internal volume ratio is selected among compressor specifications of low, intermediate, and high compression ratio depending on uses. The selected compressor achieves maximum polytropic efficiency under a certain operating condition, i.e. at a certain compression ratio, and plytropic efficiency decreases at compression ratios other than that. This is for the wasteful work needed to be done when the compressor is operating at the compression ratio other than the compression ratio corresponding to the internal volume ratio of the selected compressor, because a pressure difference is developed between the pressure in the discharge space and that of the gas to be discharged into said space from the compression space formed by a pair of rotors of the compressor.
Screw compressors capable of manually adjusting internal volume ratio with the operation of compressor stopped are widely used, and there are also screw compressors capable of automatically adjusting internal volume ratio, but generally a screw compressor is provided with an unloader valve for varying the volume of the gas to be sucked, so its structure inevitably becomes complicated if the function of adjusting internal volume ratio is added. Therefore, generally, the internal volume ratio of a screw compressor provided with an unloader slide valve for controlling gas flow rate is not controllable, and it is difficult to always attain high polytropic efficiency. As mentioned above, the internal volume ratio of a screw compressor is selected or manually adjusted with the compressor stopped or rarely automatically controlled.
FIG. 6 illustrates schematically the general structure of a conventional screw compressor. In the drawing, as a male rotor 12 and a female rotor (not shown in the drawing) meshing with the male rotor rotate, gas is sucked from an inlet port 15 into the space formed by the meshing tooth faces of both rotors and the inner peripheral wall of a rotor casing 14 (hereafter said space is referred to as the space between teeth). As the rotors rotate, the volume of the space between teeth increases, for the meshing line of the tooth faces moves toward the discharge side. When said volume becomes maximum, the communication of the space between teeth with the inlet port is shut off, so the space between teeth is closed, and the sucked gas is enclosed in the space between teeth.
As the rotors rotate further, the meshing lines of tooth faces (preceding and succeeding seal lines of tooth tips) move toward the discharge side, the volume of the enclosed space between teeth reduces, and the gas therein is compressed. When the tooth tips of both rotor (in FIG. 6, only the tooth tip of the male rotor is shown) reach the beginning edge 17c of the cut-off part 17b formed in the discharge side of an unloader slide valve 17 (actually, the beginning edge 17c consists of two beginning edge lines each parallel to the tooth tips of the male and female rotors), the enclosed space between teeth communicates to a discharge port 16, and the gas in the enclosed space is discharged as the rotors rotate.
Internal volume ratio is the ratio of the maximum enclosed space between teeth volume to the volume of the enclosed space just before the beginning of discharge.
Capacity control of varying the flow rate of gas of the screw compressor is effected by sliding the unloader slide valve 17 which straddles the perimeters of the male rotor 12 and female rotor (not shown in the drawing) to form a part of the internal wall surface of the rotor casing 14. A slide valve stopper 20 which is configured such that it forms a part of the internal wall surface of the rotor casing 14 similarly as the unloader slide valve 17, is provided at the suction side.
When the unloader slide valve 17 is moved to left so that its right end 17a comes to a location shown by a chain line 17a′, a gap is developed between the right end 17a′ of the unloader slide valve and the stopping face 19 of the slide valve stopper 20. As a result, the space between teeth is communicated with the inlet port 15 by way of a passage (not shown in the drawing) communicating with the inlet port 15, and the gas in the space between teeth is returned to the inlet port side as the rotors rotate.
The space between teeth moves toward the discharge side as the rotors rotate and compression of the gas in the space between teeth begins when it is shut off by the right end 17a′ of the unloader slide valve 17. That is, the beginning of compression is controlled by the position of said right end 17a′. Therefore, the more the unloader slide valve is moved toward left, the lesser the flow rate of the gas becomes.
The conventional compound compressor consisting of stages of a lower and a higher pressure stage, each stage being composed to have a concentric axis of rotation and provided with an unloader slide valve, is often operated with the unloader slide valve of the higher pressure stage fixed always at 100% load position, i.e. at maximum flow rate position except when starting the compressor. Compound compressors like this have been used in many cases with low suction pressure and high compression ratio.
The compound compressor like this can be operated with high efficiency under constant high compression ratio condition, but when the compressor is operated with decreased compression ratio due to increased suction pressure or decreased discharge pressure, the operation condition of each of the low and high pressure stages deviates from the condition with which maximum efficiency is achieved. As a result, the compressor is operated with decreased efficiency and bearing load may increases resulting in decreased bearing life.
The lower pressure stage compresses sucked gas at the compression ratio corresponding with the design internal volume ratio determined in the design stage of the lower pressure stage and discharges the compressed gas to the inlet side of the higher pressure stage.
The higher pressure stage compresses the gas discharged from the lower pressure stage at the compression ratio corresponding with the design internal volume ratio determined in the design stage of the higher pressure stage.
Therefore, the suction pressure of the higher pressure stage (intermediate pressure) depends on the ratio of the volume of the enclosed space between teeth of the lower pressure stage when discharge from the space begins to the volume of the enclosed space between teeth of the higher pressure stage when compression begins, i.e. the volume of the maximum enclosed space between teeth of the higher pressure stage.
To be more specific, if the volume of the enclosed space between teeth of the lower pressure stage when discharge begins is smaller than the volume of the enclosed space between teeth of the higher pressure stage when compression begins, the gas discharged from the lower pressure stage is enclosed in the space between teeth which is larger than the space between teeth of the lower pressure stage when discharge begins, so that the pressure of the gas when compression begins in the higher pressure stage is lower than that when discharged from the lower pressure stage. That is, the intermediate pressure (suction pressure of the higher pressure stage) becomes lower than the discharge pressure of the lower pressure stage. Therefore, the gas discharged from the lower pressure stage expands in the space between the lower pressure stage and higher pressure stage, that means that the lower pressure stage compressed the gas excessively high and did wasteful compression work, resulting in decreased efficiency of the lower pressure stage.
Now if we call the ratio (the volume of the enclosed space between teeth of the lower pressure stage when discharge begins)/(the volume of the enclosed space between teeth of the higher pressure stage when compression begins) as displacement ratio, the smaller the displacement ratio, the lower the intermediate pressure becomes.
Said displacement ratio is different depending on the combination of specifications of the lower and higher pressure stages.
As mentioned before, generally the unloader slide valve of the higher pressure stage may often be fixed at the maximum capacity, i.e. at the maximum flow rate of gas. In this case, when the flow rate of gas of the lower pressure stage is decreased by capacity control, i.e. by adjusting the position of the unloader slide valve, the suction pressure of the higher pressure stage (intermediate pressure) decreases, for the more the flow rate is decreased, the smaller the displacement ratio becomes.
The discharge pressure of a screw compressor is (Vi)m times the suction pressure, where Vi is internal volume ratio, and m is polytropic exponent. Assuming polytropic exponent m is 1.3, when design internal volume ratio is 2.5, discharge pressure is 3.29 for suction pressure of 1.0, 4.94(=3.29×1.5) for suction pressure of 1.5, and 6.58(=3.29×2) for suction pressure of 2. If these discharge pressure of the lower pressure stage are the suction pressure of the higher pressure stage, and assuming polytropic exponent m is 1.3 and design internal volume ratio is 2.5 also in the higher pressure stage, discharge pressure of the higher pressure stage is 10.8, 16.2, and 21.6 for suction pressure of the lower pressure stage of 1, 1.5, and 2 respectively.
As described above, when the suction pressure of the lower pressure stage increases, the discharge pressure of the higher pressure stage increases considerably, and there happens the case that the discharge pressure exceeds the limit pressure permissible for the higher pressure stage.
When the displacement ratio is small, the intermediate pressure, i.e. the suction pressure of the higher pressure stage becomes lower than the discharge pressure of the lower pressure stage (the pressure in the enclosed space between teeth just before discharge begins), but even so, the discharge pressure of the higher pressure stage may happen to exceed the permissible pressure when suction pressure (the suction pressure of the lower pressure stage) is highly increased. The larger the design internal volume ratio is, the stronger this tendency is.
The reduction in efficiency at decreased flow rate can be evaded if the unloader slide valve of the higher pressure stage is controlled together with that of the lower pressure stage, but it is difficult to accommodate the fluctuation of suction pressure, despite that the controlling becomes complicated. To be more specific, the apprehension that particularly the discharge pressure of the higher pressure stage becomes excessively high when suction pressure increases can not be dismissed and that enough compression pressure can not be attained when suction pressure decreases.
By solving the problem mentioned above, a multi-stage screw compressor unit with two stages of a lower pressure stage and a higher pressure stage integrated can be provided, which can be operated always with high efficiency.